1. Field of the Invention
This invention relates to an apparatus for producing in a body of water acoustic impulses which are useful, for example, for seismic exploration.
2. Description of the Prior Art
Generators with pistons for producing in water acoustic impulses are known--see, for example, U.S. Pat. Nos. 3,369,627, 3,642,090, 3,642,089, 3,711,824, 4,131,178, 4,185,714 and French Pat. Nos. 2,307,999 and 2,308,112.
Pneumatically-operated, air-releasing generators (known as "air guns") for producing acoustic pulses in water are also known. Such known air guns are too heavy and bulky. They waste a considerable amount of energy of releasing compressed air into the water.
In U.S. Pat. No. 4,185,714 is described an acoustic generator for producing acoustic impulses by injecting a liquid jet into a liquid body. The generator includes a main housing having a first bore, a bottom stop wall having a main port, and a top stop wall having a top port. The generator has a single shuttle made up of a main piston, an auxiliary piston, and an interconnecting push rod. The main piston is slidably mounted in the first bore which defines, in use of the generator, a slug chamber between the bottom stop wall and the main piston. A liquid slug is confined in the slug chamber. A vent chamber is formed between the main piston and the top stop wall.
The second piston is slidably mounted in a second bore which also provides a return chamber between the second piston and a fixed seal ring which slidably receives the push rod. A trigger chamber, between the second piston and the housing, has a vent and a pressure inlet. A reservoir chamber has a pressure inlet and a pressure outlet. The return chamber has a pressure inlet. The vent chamber has a vent hole.
A normally-closed, mechanically-operated valve couples an air pressure source to the inlet of the reservoir chamber. The return chamber is directly coupled to the air pressure source. A normally-closed, electrically-operated valve couples the outlet from the reservoir chamber to the inlet of the trigger chamber. A plunger is movably mounted in a wall of the housing above the trigger chamber.
In use of the generator, the plunger becomes displaced by the auxiliary piston when it reaches its uppermost position in the second bore, thereby opening the mechanically-operated valve which admits air pressure to the reservoir chamber.
The electrically-operated valve admits air pressure from the reservoir chamber to the trigger chamber, thereby causing the shuttle to execute a forward stroke which expels the liquid slug from the slug chamber. The liquid slug forms a liquid jet that is abruptly injected through the generator's main port into the surrounding liquid body. In response to the air pressure in the return chamber, the shuttle executes a return stroke, and the cycle is repeated.
In this known seismic generator, ambient air is first compressed by the compressor and the resulting increase in gas temperature involves a loss of energy in heat exchanges between the medium outside of the generator and the compressor. Also, the compressed gas released into the ambient represents a considerable loss of energy and may even create undesirable noise.